Summary of the Invention
The present invention relates generally to a laser printing system using a rotating polygon to scan a modulated beam across a photosensitive surface, and, more particularly, to a programmable drive means for changing motor speeds in response to changes in image resolution requirements.
Electronic copiers or printers of the type which use a raster output scanner (ROS) to form images on a moving photosensitive medium are well known and commercially available. In a typical system, a gas or laser diode is pulsed and its output modulated in accordance with data information to be printed on a photosensitive medium. The modulated laser beam output is directed to a rotating optical scanner, conventionally a multi-faceted polygon element, driven by a motor, which scans the modulated beams across the medium, which can be, for example, a charged photoreceptor belt or drum. The modulated scanning beam exposes the belt or drum in an image-wise pattern which can subsequently be developed and a developed image is then transferred to an output sheet.
In the ROS scanning system, the motor drive for the scanning polygon is a critical component since output copy quality is directly proportional to the smoothness of motor operation. It has been found that for accurate speed control at relatively high speeds, an AC hysteresis synchronous motor for some systems, is preferable to the DC drive motors which are conventionally used to drive scanning polygons at lower operational speeds. The AC motor can be driven by applying either sine wave signals, or square wave signals to the motor windings. U.S. Pat. No. 4,349,847 is representative of a scanning system in which a hysteresis synchronous polygon motor is driven by sine wave inputs. The main disadvantages of the sine wave drive is that it must be enabled in combination with a tuned circuit. And, since the motor is an inductive load, it is necessary to use power factor correction capacitors across the load; necessary, because the drive transistors driving the motor are operating in the active region, and dissipate too much power with a purely inductive load. The power factor correction capacitors are chosen to have a value dependent on the frequency of the sine wave inputs. If the frequency changes by more than a small amount, the capacitor value must be changed to compensate, else the drive transistors will fail. Since these capacitors are relatively large, accommodating more than a small range of motor speeds becomes very difficult.
Use of square wave input signals to the motor does not require use of the power factor of correction capacitor, since the drive transistors are operated only in the cutoff and saturation regions, where power dissipation levels are low. The disadvantage of using the square wave drive is that the motor tends to jump out of sync at the wave front transitions, and the power must therefore be adjusted at each desired frequency change in order to maintain smooth motor operation. Thus, while square wave drives have been used in scanning systems to compensate for instantaneous speed variations known as "hunting" (see U.S. Pat. No. 4,140,903) it has heretofore not been known to drive a polygon motor with square wave signals with various, relatively large, frequency input changes to obtain a desired speed change. The advantage of a square wave drive system with proper power adjustment is that the ROS can be operated so as to enable a continuously variable resolution of the scan beams at the photosensitive medium in a slow scan direction, (the direction of the photoreceptor motion) with good copy quality.
According to a first aspect of the present invention, a square wave polygon motor drive system is disclosed in an "open loop" circuit in which a crystal oscillator is used to provide two stable reference square waves. The frequency of the square waves is divided down and a 90.degree. phase shift established. A divide by n counter provides the divided down frequency signals to provide the final square wave outputs 90.degree. out of phase and at a power level determined by a programmable power supply. According to a second aspect of the invention, automatic power adjustment is provided by providing a look-up table that maps the desired motor RPM into a power level for an optimum motor performance.
Summarizing both aspects of the invention, the drive signals (phases 1 and 2) to the motor need to be changed in frequency and in amplitude to obtained desired motor performance.
More particularly, the present invention relates a motor speed control system for controlling the rotational speed of a polygon adapted to scan a reflected beam of radiation over the surface of a photosensitive scanning medium, said system comprising:
an AC hysteresis synchronous motor coupled to said polygon and adapted to drive said polygon at said rotational speed,
means for varying the frequency of square wave signal applied to said motor in response to signals representing changes in polygon rotation speed and
means for adjusting the power level of said square wave signals in response to said square wave signal frequency variations whereby the frequency and amplitude of said square wave signals are varied to provide smooth motor speed transitions at the square wave transmission points.